HTRI learning - Question and Answers

[muzaffar84]

Guys,

A forum fellow, who is new to HTRI, asked me a question (in personal message) regarding an issue. I got an idea that we should start a new thread for such questions. So that others can also get benefit of such discussion. I'm also a learner of HTRI, so it would be beneficial for me as well.

So here is the 1st question I got from joly165

I have a query. For the first time I'm designing a heat exchanger, I'm using HTRI software v 5.0 and then run the program I get the following warning:


"THE PHYSICAL PROPERTIES OF THE HOT FLUID HAVE BEEN EXTRAPOLATED BEYOND THE VALID TEMPERATURE RANGE. CHECK CALCULATED VALUES"

" AN INTERNAL TEMPERATURE CROSS EXISTS IN HE EXCHANGER. THE PROGRAM HANDLES THE REVERSE HEAT FLOW PROPERLY IN THE CALCULATIONS, BUT YOU MAY WANT TO CONSIDER CHANGING THE TERMINAL PROCESS CONDITIONS TO AVOID THE INTERNAL TEMPERATURE CROSS"


I would like to know how to avoid crossing temperature, so I would not get this warning again...

[muzaffar84]

My reply:

Your heat exchanger's tubeside or shell side is 2 pass, right? and your hot fluid outlet temp and cold inlet temp have little difference. You should either increase temp difference or try making shell side/tubeside 1 pass.

[muzaffar84]

joly 165 replied:

thanks a lot,

try it that way and no longer present temperature crosses but now I have problems with the overdesing of which appears in -94.8%, which failed to raise, you know how I can increase it????

pd. and yes i heat exchanger´s 2 pass for tubeside, and i have little difference between hot fluid oulet temp and cold inlet temp (2 ºF)

[muzaffar84]

All previous posts were a copy paste. Here is my original reply to above question.

You're welcome. Well, increasing temperature difference will help you increase your Overdesign factor (which should be 2% to 10%). Always there is one side a desired output (either cooling or heating a fluid). We can adjust other side (e.g. a hot water supply in shell side to increase the fluid temp on tubeside). To adjust this you should leave the flowrate entry blank. HTRI will automatically adjust the flowrate as per your other process data. But you should also be careful about pressure drop and flow induced vibration (Very high flow rate will result in increased pressure drop n flow induced vibration). If it's not possible further to increase the flow rate. The only option left is to increase the no. of tubes or tube length hence increasing the shell geometry as well.

I'll highly appreciate the comments of experts of thermal design. I, once again, would like to mention that I'm also a learner in HTRI

[sniperel]

How can you simulate using HTRI a heat exchanger having for one side 2 immiscible phase(hydrocarbon-water)?

[aseptman]

Property Extrapolation:
HTRI makes use of VMG Thermo data internally. (You can also input/provide data though)
The VMG data is available for certain ranges and if your design range is beyond internal range then you get extrapolation warning. It is OK most of the times.
Temperature Cross:
Any exchager will have a Hot and Cold fluid. Thus you have Hotin/Hot out and Cold in/Cold out temps. You calcuate approches at inlet and out let based on these 4 t's. The Delta LMTD is a log ave of the approches.
Now when you have a cross in the saidT,s ( eg Hot in= 100, Hout out is 35 and Cin is 32 and out is 40 then you have a cross at outlet or inlet depends. When you have cross it is better to use single pass especially when there is a phase change
% Overdesign:
% oD of -94% means a underdesigned excahgers. Try rating mode and use a geometry that gives area approx double than what you have till you get desired overdesign.
I hope this clarifies your issue.

[muzaffar84]

How can you simulate using HTRI a heat exchanger having for one side 2 immiscible phase(hydrocarbon-water)?

It's simple. You can define all components of your stream in HTRI, if you are selecting "compnent by component" as your physical properties input. Just you need to enter component fractions and ofcourse physical properties of all components (for both phases where exists). You can also use HTRI's internal phical properties for standard fluids. If you have the stream/mixture properties, better you select "mixture properties via grid" in physical properties input.

[asif]

what will be the effect of total gravity flow regime on shell side condenser performance, how we can shift from total gravity flow regime to mixed flow regime i.e shear , transition, gravity. if shell side is non baffled then there is need to add the support plate to prevent induced vibration, my question is this when you select baffle option as NON then how you can add support plate and its appropriate number on geometry panel

[viswanathankasi2]

good thread what u have started . it will be very useful for eveyone good luck

[nilam]

Dear All,
can anybody give me the checklist for shell & Tube Heat Exchanger design?
I need it badly.

[nilam]

Hi!
How to design the Air Cooled Heat Excahnger when Tube side outlet temp. is less than the Air inlet temp. ? As per HTRI help menu, i have tried with Economiser mode but found the same error msg. : Process out temp. is less than the air in temp. and run failed.

Kindly advise.

[sniperel]

Hy! I guess you need to change the outlet tube temperature or if can't changed try to put an water cooler.